This invention relates to an image forming apparatus that forms an image using electrophotographic technique.
An image forming apparatus that forms a color image using electrophotographic technique includes a plurality of photosensitive drums and corresponding transfer units. The photosensitive drums and transfer units face each other and are disposed along a feeding belt for feeding a recording medium. Toner cartridges are provided for supplying toners of respective colors to the respective photosensitive drums. A latent image is formed on the surface of each photosensitive drum by means of a charging roller and an exposure unit disposed on the circumference of the photosensitive drum. The latent image is developed by the toner, and a toner image is formed on the surface of each photosensitive drum.
When the feeding belt moves, a recording medium is fed through between the respective photosensitive drums and the transfer units. Each transfer unit is applied with a transfer output (i.e., a transfer bias), and the recording medium is applied with an electric charge opposite in polarity to the toner image formed on the surface of the photosensitive drum. The toner image is transferred from the surface of the photosensitive drum to the recording medium. Thereafter, the recording medium is fed to a fixing unit. The fixing unit applies heat and pressure to the toner image, so that the toner image is fixed to the recording medium.
Conventionally, a “differential constant current controlling method” is known as a method for controlling the transfer output applied to the transfer unit. In the differential constant current controlling method, a target current flowing through a transfer unit is predetermined, and the transfer output applied to the transfer unit is determined while detecting the feedback current so that the feedback current becomes equal to the target current. Further, the transfer output is controlled in accordance with the preliminarily detected electric resistance of the transfer unit.
Therefore, Japanese Laid-Open Patent publication No. 2000-235316 discloses a technique in which an amount of variation (i.e., variation with time) of the electric resistance of the feeding belt is detected, and the transfer output is controlled according to the detected amount of variation.
However, in the above described technique, the detection of the variation of the electric resistance of the feeding belt is performed irrespective of whether there is a variation of the temperature or not. Therefore, there is a problem that it takes a long time to start printing operation.
Accordingly, it is demanded to develop an image forming apparatus capable of controlling the transfer output quickly and correctly in accordance with the variation of the environmental condition such as temperature, humidity or the like.